Surgical microscopes are typically covered with a disposable drape to prevent contamination of the microscope from the patient or contamination of the patient from the microscope. The drape comprises a flexible cover having a flat transparent window therein aligned with the objective lens of the microscope.
In many surgical microscopes the objective lens transmits light from a light source to the surgical site to illuminate the surgical site, and the objective lens also transmits the image of the surgical site to the optical path of the microscope.
The flat window in the drape allows light form the objective lens to pass to the surgical site, and it allows the image to pass from the surgical site to the objective lens. One problem encountered with conventional drapes is that the flat window will reflect some of the light that is transmitted from the objective lens back to the objective lens and into the optical or viewing path of the microscope. The reflected light impairs the image of the surgical site due to a phenomenon called halation. Anti reflective coatings can be applied to the window, but even with antireflective coatings, a substantial amount of light is reflected into the optical or viewing path. U.S. Pat. No. 5,608,574, incorporated herein by reference, discloses a flat slanted window. Tilting the window can also reduce the amount of light reflected into the optical or viewing path, but this introduces a distortion or astigmatism into the viewed image. (An astigmatism is a defocusing of the image along one axis). Also the slant of the drape lens reduces the clearance between the microscope and the surgical field, which may cause the surgeon to compromise and use a longer lens to allow adequate working space between the microscope and the surgical field. Some surgeons remove the window to avoid these problems, compromising the effectiveness of the drape.
Another problem experienced with flat windows is that to save cost these windows are made of molded plastic, and flat molded plastic parts tend to have irregular peaks and valleys on their surfaces. This surface irregularity introduces additional distortion into the image, and because it is essentially random, it is difficult for the eye to accommodate, resulting in a very poor image.